//执行上下文 var和let融合
function x(){
    var a = 1;
    let b = 2;
    {
        let b = 3;//执行后销毁，维护一个栈
        var c = 4;
        let d = 5;
        console.log(a);
        console.log(b);
    }
    console.log(b);//2
    console.log(c);//4
    console.log(d);//error
}
x();